In many wireless communication networks Reference Signals RSs for demodulation of control and data signals are needed in order to obtain good performance. In LTE the so-called Cell-specific Reference Signals CRS were originally used as demodulation RSs for both the PDSCH Physical Downlink Shared Channel, i.e. the data channel, and the PDCCH Physical Downlink Control Channel, i.e., the control channel. The PDCCH is used to transmit a downlink transmission assignment and an uplink transmission assignment (which is typically called an UL grant) from an eNB to an User Equipment UE where the downlink assignments assigns a transmission on the PDSCH destined to the UE. It is noted that the CRS may also be used for mobility purposes in that the wireless device may perform mobility measurement using the CRS.
Further, pre-coding is a well-known technique to adopt a transmitted signal so that the received signal becomes stronger in a decoding sense. The LIE standard quite early supported closed-loop pre-coding wherein the UE reports a suggested pre-coder for the PDSCH. In TM4 Transmission Mode 4 the PDSCH is demodulated using CRCs that are not pre-coded, i.e. at least not dynamically. This means that the pre-coder used by the eNB for the PDSCH transmission needs to be communicated to the UE. The solution is that pre-coder used is signaled inside the Downlink Control Information DCI send over the PDCCH.
Further, in Rel-10 of the 3GPP Third Transmission Partnership Project specifications TM9 Transmission Mode 9 was introduced, and demodulation of the PDSCH was using new reference signals that where pre-coded in the same way as the PDSCH. These reference signals are called Channel State Information-Reference Signals (CSI-RS). With TM9 there was hence no longer a need to explicitly signal to the UE what pre-coder was used.
Further, in Rel-11 the Evolved PDCCH EPDCCH that enabled dynamic pre-coding of also the channel used for sending assignments, i.e. DCIs, to the UE. However, the EPDCCHs has the restriction that the demodulation reference signals are common for all the EPDCCHs meaning that the pre-coding cannot be optimized for each UE receiving an EPDCCH since all the EPDCCHs are using the same demodulation RS.
However, when advancing to 5G radio technology, it is foreseen that the CRS and CSI-RS will not be continuously sent by the radio access network, in order to enable the radio access network to shut down individual network nodes of the radio access network and by that to save energy. This may come along with the problem that the currently known concept of mobility measurement might not be compatible with the foreseen 5G concept.